According to a known diesel particulate filter (referred to as DPF hereinafter) disposed downstream of a diesel engine to trap particulate matter (referred to as PM hereinafter) in the exhaust gas from a diesel engine, the trapping functions of the DPF are recovered or regenerated by burning the trapped PM.
In order to regenerate the DPF, the bed temperature of the DPF must be increased to a burning temperature of the PM. This has been accomplished by increasing the temperature of the exhaust gas to about 600° C. to 650° C. by control such as increasing the fuel injection quantity, throttling the intake air, retarding the fuel injection timing, or the like. A temperature of 600° C. to 650° C. is necessary to burn the soot in the PM.
However, because the fuel injection quantity and the fuel injection timing deviate from appropriate levels determined on the basis of the engine operating conditions (or the driving conditions in the case of an engine operated in a vehicle), during the course of regenerating the DPF as described above, there arise problems of decreased output and decreased fuel consumption.
In order to overcome these problems, Japanese Unexamined Patent Application Publication No. 2003-269221 (JP '221) provides the following technique.
First, before starting DPF regeneration, after-injection is conducted to increase the ratio of the soluble organic fraction (SOF) composed of unburned hydrocarbons (HC) and oils in the PM. The SOF is present in the exhaust gas by adhering on the surface of the soot or the like and becomes trapped in the catalyst unit as the exhaust gas passes through the catalyst unit.
The SOF has a relatively low burning temperature and easily starts burning by oxidation reaction in the catalyst unit.
Utilizing the burning property of the SOF, the exhaust gas temperature is increased until the temperature reaches the burning temperature of the SOF, and the temperature is subsequently allowed to increase to the burning temperature of the soot using the heat of combustion from the SOF.
However, using the SOF as a heat source as described in JP '221 has the following problem. That is, if the exhaust gas temperature is increased up to the range of about 300° C. to 450° C. as a result of driving conditions of the engine, the SOF on the soot is removed, and there may not be sufficient SOF to increase the exhaust gas temperature to a temperature that burns the soot. Moreover, if an oxidation catalyst is disposed upstream of the DPF, the SOF is removed by the oxidation catalyst and does not reach the DPF. In other words, the SOF may not adhere to the soot in the DPF.